In most prior art solid state commercial gas sensors, it is necessary to heat the sensor element to elevated temperatures in order to acquire both fast response time and high sensitivity to objective gases. For example, N-type semiconductor tin oxide gas sensors and catalytic combustion type Pd/Pt gas sensors must usually be operated in a temperature range of ca. 200.degree. to 500.degree. C. These sensors must be equipped with heaters connected to external power sources. Therefore, room temperature CO gas sensors, which use less power, are desirable.
It is well known that CO reacts with moisture in air at room temperature, and forms protons, electrons, and CO2 in an oxidation reaction of CO. EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +2H.sup.+ +2e.sup.- ( 1)
It is also known that there is a moisture formation reaction by combining protons, electrons, and oxygen in a reduction reaction of oxygen: EQU 2H.sup.+ +2e.sup.- +1/2O.sub.2 .fwdarw.H.sub.2 O (2)
These two reactions are the basis of prior art room temperature low power electrochemical gas sensors utilizing a proton conductor. FIG. 1 shows the transport processes of such a CO gas sensor. A protonic conductor 12 conducts ionized hydrogen atoms from a sensing electrode 16 where the sensor signal originates from the oxidation reaction of carbon monoxide at sensing electrode 16. Ionized hydrogen atoms, each of which constitutes a single proton, are conducted through protonic conductor 12 to a counter electrode 14. Electrons that are liberated in the oxidation of carbon monoxide at sensing electrode 16 are conducted through an electrical lead 22 to voltage meter 18, through an electrical lead 20, and to counter electrode 14 for a reduction reaction of oxygen. In a steady state reaction, the hydrogen ions are transported from sensing electrode 16 to counter electrode 14 in the depicted potentiometric CO gas sensor.
The current generated by the reactions depicted in FIG. 1 can also be measured by an amp meter 24 having a resistor R.sub.L 26, which circuit represents a transport process of an amperometric CO sensor. Absent amp meter 24, resistor R.sub.L 26, the leads thereto which are shown in phantom, transport processes of a potentiometric CO gas sensor are shown for voltage meter 18 and leads 20, 22.
Whether the transport processes shown in FIG. 1 are for potentiometric CO gas sensor or for an amperometric CO sensor, electrons from the process of the oxidation reaction of carbon monoxide travel as seen in arrow 21 in FIG. 1 through leads 20, 22.
The sensor of FIG. 1 is operated in a current mode when the sensing and counter electrodes 16, 14 are connected to each other through load resistor R.sub.L, or are connected to a DC power source (not shown) which electrically drives the protons across proton conductor 12.
A prior art room temperature proton conductor sensor developed by General Electric using a polymer porous support material saturated by a liquid proton conductor, has been constructed as an electrochemical amperometric CO gas sensor (the G. E. Sensor). In the G. E. Sensor, a liquid reservoir was used to provide the liquid proton conductor to the porous support material. Protons, which are indicative of the ambient CO concentration, were driven across the porous support material through the liquid conductor by a DC voltage. Electrical current response of the sensor to ambient CO concentration was linear. The cost of the sensor with such a complicated design, however, is high and is thus not be suitable for practical consumer applications.
In U.S. Pat. No. 4,587,003, a room temperature CO gas sensor using a liquid proton conductor is taught. Basically, the mechanism and design of the sensor were similar to the G. E. sensor, except that the outside surfaces of the sensing and counter electrodes of the sensor in this patent were coated by porous NAFION.sup.TM layers. The CO room temperature gas sensor taught in the patent currently costs about $200.00. The lifetime of such a sensor is about 6-12 months due to the rapid drying of the liquid of the electrolytes. In addition, the sensor requires maintenance due to leakage and corrosion of liquid electrolyte.
The discovery of room temperature solid proton conductors aroused considerable efforts to investigate low cost, all-solid electrochemical room temperature CO gas sensors. One such sensor that was developed was a room temperature CO gas sensor with a tubular design using proton conductors, electronically conductive platinum or the like as the sensing electrode, and electronically conductive silver, gold, graphite or the like as the counter electrode. The sensing electrode decomposed carbon monoxide gas to produce protons and electrons, whereas the counter electrode exhibited no activity to decompose carbon monoxide with the result that a Nemst potential occurred between the two electrodes. Thus, carbon monoxide gas was detected.
In detecting carbon monoxide with the tubular design sensor, protons and electrons are generated at the sensing electrode. For the reaction to be continued, protons and electrons must be removed from the reaction sites, and CO and moisture must be continuously provided from the gaseous phase to the reaction sites. Therefore, the CO reaction only occurs at three-phase contact areas. The three-phase contact areas consist of the proton membrane phase, the platinum electron phase, and the gas phase. Due to the limited three-phase contact areas in the tubular design sensor, the CO reaction was slow. Additionally, the response signal was weak. Further, the Nernst potential was not zero in clean air.
A modified electrochemical CO room temperature gas sensor using a planar or tubular sensor design was a subsequent development to the earlier tubular design CO sensor. In order to overcome the problem that the Nernst potential is not zero in clean air experienced with the earlier tubular design CO sensor, the improved design proposed a four probe measurement method for CO gas detection. The improved design achieved a zero reading in clean air, and the improved sensor was insensitive to variations in relative humility. Theoretical analysis based on electrochemistry, however, indicates that there is no difference between the four probe method and the normal two probe method of the earlier tubular design CO sensor. The improved sensor still used electronic conductors for both the sensing and counter electrodes, and showed slow and weak response signals to CO gas.
A still further improved design of a CO sensor is a room temperature electrochemical gas sensor using a solid polymer proton conductor with a planar sensor design. Response of this further improved sensor to CO was very weak, and was in the nA range even as a DC power source was applied. Apparently, the internal resistance of the sensor was too large. Calculations based on this further improved sensor dimensions indicates that the ionic resistance of the proton conductor membrane is about 400 K-ohm, which is too large to generate a usably strong signal. Further development and improvement of the planar CO gas sensor, which incorporated a sensing mechanism, resulted in performance that was still in nA range of sensor response.